Traditionally, at the presentation of the Fields medals at the ICM opening ceremony, the composition of the Fields medal committee is disclosed. This information can be found in the first volume of the respective ICM Proceedings (for older ICM, somewhat hidden in the presentation speech of the chair). That is, except for the 1962 ICM at Stockholm, the first for which the Fields Committee was appointed via IMU). It is only clear that Nevanlinna (who gave the presentation speech) was the chair:
After an interval, Professor Nevanlinna in his capacity of chairman of the Fields Medals Committee, read the report of the Committee which was as follows: [..] The problem of suggesting names for the award of the Fields medals has, since the last congress, been entrusted to the International Mathematical Union. To prepare the names at this Congress in Stockholm, the Union appointed a Fields medals committee. This committee has the honour to make known its decision here.
Some more details are given in the Secretary's report:
The scientific programme was drawn up in close cooperation with the International Mathematical Union, which for this purpose nominated a Consultative Committee with Professor
G. de Rham as chairman. The Swedish representatives in charge of the scientific programme were Professors L. Gårding, L. Carleson and L. Hörmander. Thus a first meeting was held in Zurich in November 1960 followed by a meeting in Düsseldorf in January 1961. As a result of these two meetings the "International Fields Committee", that elects the two prize winners, was constituted. Furthermore a list was made of speakers to be invited to deliver one-hour addresses on chosen topics in different fields of mathematics.
From this passage, one might guess that there was an overlap of the Fields and the Program Committee (which would be a historical exception), but I couldn't find more explicit information in the volume. Does someone know more about the composition of the committee?